In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each having the same frame length. The radio frames are numbered sequentially with a System Frame Number (SFN). Before a UE can access a serving base station, the UE needs to synchronize with the radio frame format used by the serving base station. Both physical (PHY) layer synchronization and Media Access Control (MAC) layer synchronization are performed.
For example, upon power-on, a UE first acquires downlink (DL) synchronization and adjusts its timing, frequency, and power via a synchronization channel (SCH) broadcasted by a serving base station. After DL synchronization, the UE acquires uplink (UL) PHY layer synchronization via ranging procedures and MAC layer synchronization via network entry procedures with the serving base station. During the synchronization process, the UE can detect radio frame edges/boundaries, and read SFN numbering information from radio signals transmitted from the base station, including other information such as frame format and frame structure.
The radio frame formats of different base stations, however, may not be synchronized with each other. In addition, although the frame lengths are typically the same, the frame structures of different base stations may be different, depending on the SFN. Therefore, in addition to determining frame format, the SFN may be required by the UE to communicate with the base station. For example, during a random access procedure, a radio resource allocated for a random access channel (RACH) to the UE may be related to the SFN.
The lack of synchronization between different base stations' frame formats poses problems for handover operation. A UE that is synchronized with its current serving cell cannot communicate with a target base station (e.g., a target cell) until the UE can synchronize with the frame format of the target cell. More specifically, the UE needs to know the SFN of the target base station when performing initial transmission on the RACH. This is because that the radio resource (e.g., the RACH opportunities) for RACH signal transmission may be related to the SFN of the target cell.
One mechanism to obtain the SFN is for the UE to read the physical broadcast channel (PBCH) or the broadcast channel (BCH) of the target cell. The UE can implicitly determine the SFN from decoding the PBCH/BCH. The PBCH/BCH is repeated every 10 ms, meaning that it takes an average of 5 ms and takes 10 ms at the worst case to receive and decode the PBCH/BCH. Such delay increases the handover interruption time. It is thus desirable that the UE can have other mechanisms to obtain the SFN of the target cell for handover operation before PBCH/BCH reading.